1. Field of the Invention
The present invention relates to an antireflection film which is provided on a surface of an optical filter, a lens, or the like, a lens, and an imaging device.
2. Description of the Related Art
Chalcogenide glass includes, for example, sulfur (S), germanium (Ge), selenium (Se), or tellurium (Te) as a major component. This chalcogenide glass is cheaper than Ge crystals, which are a material of the related art, and can be easily processed into a desired shape of an optical element by mold forming. Therefore, chalcogenide glass has been highly anticipated as an optical member such as a lens or an optical filter in which far infrared rays (8 to 14 μm (which has the same meaning as 8 μm or more and 14 μm or less; hereinafter, the expression “to” is used to express a range including boundary values) are used.
Since the refractive index of chalcogenide glass is 2.5 to 2.6, the surface reflectance is high, and the transmittance remains at about 60%. Therefore, only a simple method of processing chalcogenide glass into a shape of a lens or the like is not sufficient to obtain a sufficient imaging light amount. Therefore, in order to reduce light amount loss caused by surface reflection, an antireflection film is provided on a substrate formed of chalcogenide glass (refer to JP2014-032213A and JP2011-221048A corresponding to U.S. Pat. No. 8,535,807).
In the related art, Ge which is a constituent element of a substrate or a compound (sulfide) including sulfur (S) which is an element belonging to the same group as Se, is used as an adhesion layer to configure an antireflection film. The reason why the layer formed of Ge or a sulfide exhibits high adhesiveness with a substrate formed of chalcogenide glass is that a strong bond is formed at an interface between the substrate and the film, the strong bond being a Ge—Ge bond which is formed in a case where both the substrate and the film are formed of Ge, or a Se—S bond which is formed in a case where the substrate includes Se. In particular, the Ge—Ge bond is a covalent bond (sp3) having a high bonding strength and is more preferably used than a sulfide from the viewpoint of easiness of film formation and maintenance of the device.
A high bonding strength in a Ge—Ge bond or the like is generated in a case where an element on the film side is an element belonging to the same group as Ge. For example, a Ge—Si bond which is formed in a case where a substrate is formed of Ge and a film is formed of silicon (Si), or a Ge—C bond which is formed in a case where a substrate is formed of Ge and a film is formed of carbon (C) has a high bonding strength. However, Si is not transparent to far infrared rays (8 to 14 μm) and thus is inappropriate as a film forming material.
On the other hand, C is transparent to a far infrared region as long as it has a diamond structure (sp3), and forms a covalent bond (sp3) with Ge on a substrate side and exhibits high adhesiveness. It is difficult to form a C film (carbon film) having a complete diamond structure, but a C film having a diamond-like carbon (DLC) structure can be formed by optimizing film forming conditions. Such a DLC film is an amorphous carbon film in which a C-atom skeleton structure has both diamond sp3 and graphite sp2.
The DLC film has an amorphous structure in which, unlike in a crystalline structure, there is no periodicity in atom arrangement and bond angle or distances between atoms are irregularly distributed. Therefore, strains (internal stress) are likely to accumulate in the amorphous structure. Therefore, it was verified that, in a case where a DLC film having a thickness of several hundreds of nanometers is formed, the film is broken by internal stress. Accordingly, only a simple method of forming a DLC film on a substrate formed of chalcogenide glass is insufficient, and it is necessary to secure adhesiveness.